1. Field of the Invention
The present invention relates to improved pastes for forming transparent, electrically conductive films in any desired shape, and more specifically to pastes for forming transparent, electrically conductive films which are composed of a metal oxide and which can be firmly attached to an electrically insulating substrate such as glass or ceramic by screen-printing and calcination.
It is well known that oxide films such as those of indium, tin, antimony, cadmium and the like formed on an electrically insulating substrate such as glass or ceramic have a high transmission factor and good electrical conductivity. Hence and hence these films have been increasingly used in recent years in such fields as semiconductor elements, liquid crystal display devices, electrochromic display devices, and have also been used as electrically conductive films to prevent the freezing of water on windowpanes. Transparent, electrically conductive films of such metal oxides can be formed by various methods such as chemical spraying, vacuum deposition, dipping, screen-printing, and the like.
The chemical spraying method is useful for obtaining films having relatively large areas, but is not economical when used to obtain films of fine, complex shapes, since it requires additional steps such as etching and the like.
The vacuum deposition method, on the other hand, does not require etching because of the masked deposition method which has been developed in recent years. The films prepared by this method, however, must be produced in batches and so the method is not suitable for mass production.
The dipping method is suitable for mass production, but in this method, however, the films produced have inferior properties compared with those obtained by the above methods. Moreover, an etching operation must be carried out, which reduces the practicability of the method.
The screen-printing method is free from the above problems, and enables films of any shape to be obtained by printing and calcination, without the need of treating waste liquid produced from an etching process. However, the pastes which have been produced by this method for forming transparent, electrically conductive films do not have good stability, and the operability of screen-printing is poor. Accordingly, it is difficult to print fine stable patterns.
Organometallic compounds such as indium soaps of organic acids, e.g., indium octylate, (C.sub.7 H.sub.15 CO.sub.2).sub.3 In, having strong ionic bonds, have heretofore been used in pastes for this method. These compounds, however, readily undergo hydrolysis. When prepared in the form of pastes, they are quickly converted into gels and undergo chemical changes relatively easily. Moreover, these pastes have low stability and hence reduced operability during screen-printing. Ethyl cellulose has been used as a binder, but when it is calcined at a temperature of, for example, 500.degree. C., it does not burn completely. Accordingly, the resultant film has relatively high electric resistance and low strength.
In order to eliminate these defects, the inventors of the present invention have made extensive studies, and have discovered that the stability of the paste can be increased if an organoindium complex such as trisacetylacetonatoindium (III)[In(acac).sub.3 ] is used as the compound which forms a metal oxide film upon calcination. It may be used together with an organotin complex such as dimethyltin acetope [(CH.sub.3).sub.2 Sn(acac).sub.2 ], the latter compound being used to adjust resistance. The inventors have also discovered that a film which has increased adhesion and reduced resistance can be obtained if nitrocellulose is used as a thickening agent.
When nitrocellulose is used, however, the lifetime of the paste is shorter than when ethyl cellulose is used, which still leaves problems concerning the operability during screen-printing.
The inventors therefore have attempted to improve the properties further by using deoxidized nitrocellulose or a mixture of solvents such as butyl carbitol, butyl cellosolve, benzyl acetate, and dimethyl phthalate, in order to increase the printability. The improvement, however, was not sufficient. Specifically, a problem remains when fine patterns of a size of about 0.1 mm are screen-printed.